| Moondark for June: Romancing the Moon | |
| Unlike
amateur astronomers, horseshoe crabs do come out under the light of the
full moon. Every May and June, on nighttime spring tides, Delaware Bay
becomes the horseshoe crab capital of the world. That’s when
millions of these ancient arthropods come to our shoreline to mate and lay
eggs. Amateur marine biologists join
these living fossils to marvel at the
prehistoric oceanic ritual, and take stock of their numbers. Two to four weeks later, eggs hatch, but in
that time many are washed from the sand and become food for shorebirds.
The most notable of these is the red knot, which arrives here from the
southern tip of Argentina. Without feasting on horseshoe crab eggs,
these birds cannot make it northward to their nesting areas in the
Canadian Arctic. There are many ways the Moon affects the Earth, its ocean and marine life. Another spectacular example is the mass spawning of corals that occurs annually in synchrony with the light of the full moon. In the Caribbean, several days after the full moon in August and September, many species of coral release eggs and sperm that float to the surface in what is described as an upside-down, underwater snowstorm. Tiny planula larvae from the fertilized eggs spend varying times as plankton in the water column before settling back on the reef. Mass spawning occurs in other months elsewhere in the tropics, November and December on the Great Barrier Reef, for example, though always and predictably the same number of days after full moon when water temperatures are right. Special travel packages promote these events for adventurous SCUBA divers. It’s clearly worth asking why reproductive activities for so many animals seem keyed to the Moon. Tidal range, moonlight and temperature are the most commonly cited cues. Surprisingly, there is enough moonlight in clear, tropical water over coral reefs for even human divers to see dimly in many meters of depth. Beyond such general factors, determining exact or optimal conditions has usually proved elusive, and often triggers seem to act synergistically. Regardless of the precise cues or circumstances, aggregations and opportunity for successful mating are certainly required for continuation of all species. While life on Earth is often in phase with the Moon and the tides, there are other, less apparent effects that may be just as important in the long term. The presence of a large moon has stabilized the angle of the axis of spin to the ecliptic to within a few degrees of the current value of 23.5º. This obliquity is the angle the Sun swings north and south of the celestial equator and determines our seasons. Between these latitudinal limits defined by this angle, we have the tropical rain forests, deserts, coral reefs and deep blue oceans. At higher latitudes in temperate zones, there are dominant, seasonally forced patterns of biological productivity and abundance both on land and in the ocean. Ice permanently covers both poles, at least for the time being, and these high latitudes are colonized only by rare and specialized life forms. A great question formed the title of Neil Comins' fascinating book from years ago: “What if the Moon didn’t exist?” You can see what might be the result overhead on the next clear evening by turning your telescope to Mars, now passing through the Beehive Cluster. At present, Mars’ seasonal variations are similar to Earth’s since its obliquity is remarkably near the same value. But over the time scale of planetary evolution and climate, Mars’ axis has varied greatly from nearly vertical (and thus no seasons) to tilted some 60º. Here on Earth, without a large moon to stabilize the axis, obliquity could range from 0º to 85º, and seasonal cycles would vary wildly over geologic and evolutionary time. Without the synchrony made possible by a bright monthly moonlight and inconstant seasonal forcing, diverse communities of complex organisms may never develop. So despite being warmer and wetter, life on Earth beyond the microbial might not be possible. Although moonlight certainly interferes with our enjoyment of the skies, the Moon makes life on Earth much more interesting. So while walking along the beach in the moonlight this summer, it’s worth considering how much life on Earth owes to the Moon. Moondark is written by Douglas C. Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 25 May 2008. Text and images on this web page are free for non-commercial use with attribution under a Creative Commons Attribution-Non-commercial 3.0 License. Ask Doug about other uses. |
